I don't think I saw one but:
Exeggcute -
Low Tier
Availability: Pretty damn late... it appears in the Safari Zone, though it's fairly common and easy to catch, and comes at a max of level 26-27, which is...possibly useable... but decidedly under leveled. It can be immediately evolved with a Leaf Stone.
Stats: The evolved Exeggcutor has freaking amazing stats-- of course, it's one of the most used OU Pokemon after all. Massive fat 125 Special, and above average stats everywhere but Speed-- stats are a major reason why Exeggcutor is an amazing force in competition.
Movepool: Exeggcute comes to you with a pretty meh movepool of Hypnosis, Barrage, and Reflect, and immediately learns the similarly almost-good-but-still-meh Leech Seed. It's best Grass Stab is Mega Drain (ew...), so like competitive sets, you will likely be depending on the Psychic TM as your main means of doing damage; though you do have immediate access to the Psychic TM at the point in the game you get Exeggcute. Exeggcutor does have a decent 95 Attack stat and can use Strength, Stomp, or Hyper Beam. It gets the powders too, but really late (level 48 for Sleep Powder... ew...), so you'll likely be immediately evolving and just enduring with Hypnosis. Explosion, probably don't want to use it in-game.
Power: Psychic and 125 Special rips shit up. Mega Drain can do decent damage against opponents it hits super effectively, and Exeggcutor's decent Attack means it can actually best other Psychic-types like Alakazam by stomping them to death on their weaker physical side.
Type: Grass / Psychic is obviously good typing in general-- it's the main reason Exeggcutor is so popular in OU. However, in-game, Grass does more to hurt Exeggcutor than help, especially at the point in the game you get it. Ground-, Electric-, and Water-type attacks are rare enough that these much-sought resistances in the competitive scene are less useful in game. Low Speed and weaknesses to Fire-, Poison-, and Ice-type attacks are a bit of a burden.
Match-ups:
Koga: If you evolve it and teach it Psychic, it can destroy anything he has. Keep in mind you will be horribly outmatched level-wise (you catch Exeggcutor in Fushia at max level 27, Koga's Pokes are all level 37-50). In RB, if you don't do some major grinding, his pokes will outrun and smack you with powerful super effective Sludge attacks. In Yellow, the Venonats can only status you, but Venomoth carries the 4x super effective Leech Life (though... it is only Leech Life), and is at level 50.
Saffron Rockets: You have STAB Psychic, and resist Electric, so you will have a big advantage.
Sabrina: Exeggcutor can do well here if you have trained it sufficiently and taught it Stomp or Strength. Its Psychic resistance, high Special, and decent Attack give it a significant advantage--again, assuming you've leveled up sufficiently.
Blaine: Er... just don't get hit by a Fire-type attack. Yeah...
Giovanni: Exeggcutor obviously dominates this gym.
Lorelei: Exeggcutor's not dead weight here, with its massive Special allowing it to whether some Ice-type attacks, and retaliate with super effective Mega Drain (healing back in the process). However, your low Speed, high probability of being under leveled, and the sheer power of Lapras's Blizzard will not make this an easy match up for Eggy.
Bruno: Eggy basically laughs at anything Bruno can do (again, provided you get to a decent level).
Agatha: Everything she has will be destroyed by a STAB Psychic attack, but with your low speed, prepared to face several turns of sleeping and hurting yourself in confusion (and being extremely annoyed) before you can claim victory here... eventually.
Lance: You're slow and will not like taking repeated Hyper Beams. In Yellow, 2nd Dragonair and Dragonite can hit you with powerful super effective attacks. Not dead weight, but not great.
Rival: Exeggcutor has an advantage against most of his Pokemon. Just be wary of those with Flying and Fire attacks.
Additional Comments: All things considered, the above information definitely doesn't look bad. Good stats, a usable movepool (fuck, in RBY, STAB Psychic = usable movepool), and a few favorable match-ups. So, what's the problem? The problem is that in addition to Eggy coming so late and under-leveled in the game, it's further hindered by a really slow experience curve and low Speed. In terms of power, experience curve, and availability, you could compare it to Tauros and Staryu-- except that both of those Pokemon have a much better Speed stat. Better Speed allows them to rapidly sweep through opponents and "grind" their way to catch up to the rest of the team with comparable ease. With Exeggcutor, you will find yourself constantly stopping to bust out healing items as you try to catch up to the rest of the team-- making it decidedly more annoying to train. Given dedication, Eggy will blossom to its potential, as one of the most popular of competitive Pokemon-- in-game though, you're probably much better off with Venusaur or Victrebel... or Alakazam.
Also, everything Dracoyoshi said @Dewgong. Zapdos / Articuno are top mostly just because they appear at freaking level 50-- I mean, 50's the level you should be aiming to have your whole team at by the end of things, and to get the poke at that level is just too good. Planning to use Zapdos and Articuno on your team saves so much on training early game. If you use Snorlax, Zapdos, and Articuno, you only need to train 3 Pokemon early-mid game. So, if you pour all your effort into Alakazam, Blastoise, and Nido/Dugtrio, you should have a really high leveled and extremely powerful team with almost no effort. Though I mean, Blastoise / Alakazam / Dugtrio / Snorlax / Articuno / Zapdos is probably close to the most efficient team in terms of experience gaining and decisive power at the last stage of the game.
